A network of devices can be created by arranging a group of devices which can communicate with each other via radio frequency (RF) means to transmit data between the devices. Provided that each of the devices is within the range of the maximum communications range of each device, each device can effectively communicate with each other device in the network.
In many such networks, the devices may all be transceivers, ie are each capable of both transmitting and receiving. For the purpose of transferring an item of data, one device acts predominantly as a transmitter while other devices act predominantly as receivers.
The present invention is to be used in a “point to multipoint” system as opposed to a “point to point” system in which communications occur between only two devices at a time. In a point to multipoint communication system, communications occur between one device and two or more of the other devices in the network simultaneously.
A reliable “point to multipoint” communication system allows the creation of a shared network variable. This is a variable which is known to all of the devices in the network. For example, if one device wants to change the value of the shared network variable, it must transmit a request and be guaranteed that all devices receive and process the updated variable simultaneously. If the update cannot be made simultaneously, or not all other devices in the network receive the update, then the network does not have a shared network variable.
Shared network variables allow the creation of a network which has no central controller. All of the essential data about the operation and control of the network is known by each device in the network simultaneously. The data can be updated by any device in the network at any time and all other devices are guaranteed to update their data accordingly. This allows the control of devices within the network to be simplified, more flexible, and less costly when compared with networks having a central controller.
A single communication action between each of the devices is herein referred to as a Transaction. A Transaction occurs between a device (transceiver/transmitter) which transmits data to one or more transceiver/receivers of the data. The Transaction also includes data sent from the transceiver/receivers to the transceiver/transmitter as well as to each other of the transceiver/receiver devices in the network. In this context, a device which for a given Transaction, transmits the data to be shared throughout the network is referred to as a “transceiver/transmitter” while a device which receives that data within the Transaction, is returned to as a transceiver/receiver. It will be understood that within the same Transaction, a given transceiver/receiver may also transmit an acknowledge signal. In the next Transaction, the transceiver/receiver may become the transceiver/transmitter. When transmitting to more than one transceiver/receiver simultaneously (also known as a broadcast or multicast), it is important to know that all transceiver/receivers have successfully received the data. If even one transceiver/receiver has not successfully received the data (for example because of a bit error causing data corruption in one transceiver/receiver), then all other transceiver/receivers must be informed that not all of the other transceiver/receivers have successfully received the data.
Such networks can also optionally use a transmission system with dominant and inferior bits. This means that if there is a conflict, and two devices simultaneously transmit a dominant and inferior bit, then when monitoring the communication medium, each device will see the transmission of the dominant bit. The device transmitting the inferior bit knows that there has been a conflict and can take whatever action is appropriate. For example, this may mean the cessation of all further transmissions.
In conventional point to point communication protocols, a general procedure is to have each device transmit an acknowledge statement some time after receipt of the data. This has the drawback that the transmitting device must know exactly the number of receiving devices within the network, and how to contact each of them. The reliable transfer of the same piece of data to multiple receiving devices requires many transmissions of the same data, and a corresponding wait for each transmission to be acknowledged. The repetitive transmission of the same data to many recipients wastes the available bandwidth of the communication medium. This approach also requires the transmitter to obtain and store data about exactly which receiving devices are to accept a given transmission. This approach allows the creation of a shared network variable, at the expense of unnecessary complexity and poor use of the available bandwidth of the communication medium.
Alternatively a point to multipoint transmission can be used to transfer data to many recipients simultaneously, without any acknowledgment being returned. This renders the data transfer unreliable, and the transmitter will not be able to determine whether all of the receiving devices have successfully received the data. Unreliable transfer of data means that a shared network variable cannot be created.
In such systems it is also necessary to be able to accurately determine a time or point in a data transmission where the type of data changes (for example a change from a control data transmission timeslot to an acknowledge timeslot).
Many well-known communication protocols are asynchronous and character based. The best example is the widely used serial RS/EIA-232 port.
Such protocols can be used, with difficulty, to transmit varying and fixed time portions. However, it is important that the asynchronous interface component (referred to as a Universal Asynchronous Receiver Transmitter, or UART) have well characterised timing and delay characteristics, and that those characteristics remain invariant. For example, any variation (for whatever reason) to the inter-character gap time would be catastrophic, because it would mean that different devices would fix different times for the same critical point. There would not be a common time reference for the start of the fixed time slots.
Character based protocols should be able to carry any information item that fits into the designated character size. This conflicts with the need to have some means of signalling important points in the transmission, especially the end of the variable portion. This conflict arises because in the character based protocol, some of the characters need to take on a special meaning in order to signal the important point. This means that the special character cannot be used for data. The combined disadvantages of the asynchronous character-based protocol make it very difficult to use when a fixed time relationship is to be maintained between a variable and fixed time portion.
An alternative is to consider the protocol as a stream of bits, organised into characters by grouping (in other words, overlaying the character structure back over the bit stream). Each character can be inspected to see if it is significant or not. When a data item is to be transmitted which looks the same as a significant character, extra bits can be inserted to change it. The receiving device recognises the patterns of extra bits, and removes them, restoring the original character. By performing the recognition of the special characters at the correct point in the decoding path, both data and special characters can be separated. This is known as a bit-stuffing, and is widely used in protocols like HDLC. Bit-stuffing however, can be complicated and processing intensive.
It is an object of the present invention to provide a system and protocol for improving the communications between devices in an RF point to multipoint communications system.